A CCFL (Cold Cathode Fluorescent Lamp) inverter with its switching frequency adapted to the resonant tank frequency produces a power conversion with high efficiency and provides reliable lamp striking and open lamp voltage regulation. In a variable frequency control method, the switch is always turned on when IL crosses zero, wherein IL is the resonant current of the transformer's primary winding. However, this design approach has certain disadvantages. It produces big variations of switching frequencies when input voltage, lamp current, or when liquid crystal display (LCD) panels are changed. If the frequency variation range becomes too wide, there is potential electric-magnetic interference (EMI) between the LCD panel and the CCFL inverter.
A CCFL inverter with a fixed frequency control method does not have the EMI problem. In this method, the switching turn-on time is regulated by a clock and the switching frequency is fixed by the designed parameters. However, there is no control of the phase relationship between supply voltage and resonant current, which may cause poor crest factor, poor lamp efficiency, start-up lamp current spiking, and open lamp voltage regulation.
Accordingly, improvements are needed to utilize the advantages of both the variable-frequency and fixed-frequency control methods.